This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Dynamic exchange is being incorporated into the software for the Slowly Relaxing Local Structure (SRLS) model. This new dynamic process allows the nitroxide spin label to exchange positions between two or more sites where the spin label experiences different rotational rates and orderings. This project is motivated by our recent multifrequency study of T4 lysozyme dynamics where we have clearly differentiated two distinct conformers of the nitroxide tether. However, we previously were limited to considering the exchange to be static on the ESR time scale. Under this assumption it was found that different populations have to be employed to achieve acceptable spectral fits at different frequencies. We believe that this is a result of the different effective timescales of ESR at the different frequencies. Our new software will enable us to clarify these matters, and to obtain rates of conformational exchange.